Sensors, or transducers, may be used to detect and measure a variety of conditions including vibrations, acceleration and tilt. Sensor subassemblies are available in a variety of forms including mechanical, electrical and ferrofluidic. Mechanical vibration and acceleration sensors use pendulums or moving elements. Electrical sensors use mercury slugs or cantilever beams to make or break an electrical contact. In a mercury-based sensor, the mercury is sealed in a glass vessel. Because of a high surface tension, the mercury does not stick to the glass vessel, but responds quickly to slight vibrations. However, mercury has the disadvantage of being a hazardous material.
One type of prior art ferrofluidic sensor subassembly consists of an axially polarized permanent magnet located in a non-magnetic housing completely filled with ferrofluid as illustrated in FIG. 1 and discussed in detail in U.S. Pat. No. 4,667,414. Ferrofluid 1 is a colloidal mixture of magnetic particles suspended in a liquid carrier, such as oil. The particles are coated with a surfactant and are kept in suspension in the carrier liquid. ferrofluid 1 is sealed inside the non-magnetic housing 2, which includes a mechanism that allows the ferrofluid to expand (not shown in FIG. 1). For example, housing 2 may incorporate flexible diaphragms. A magnet 3 is free to move inside housing 2 and is suspended in the housing 2 by the ferrofluid 1, which becomes trapped in the magnetic field produced by the magnet 3.
The movement of magnet 3 can be detected by inductive coils (not shown) wound around housing 2 or by a Hall element (not shown). With the appropriate electronic signal processing, the inclination or acceleration of housing 2 can be measured.
While such a device is relatively simple and works well in many circumstances, the ferrofluid 1 is generally fairly viscous due to the presence of the oil carrier. The viscosity of the ferrofluid 1 consequently produces a viscous drag on the magnet 3 and thus slows response time of the device. Since the ferrofluid viscosity is a function of temperature, the response time is temperature dependent. Furthermore, a small increase in the mass of the magnet 3 occurs over time due to sedimentation of ferrofluid particles on the magnet surface. This affects the calibration of the device.
In order to overcome these problems, the configuration illustrated in FIG. 2 is often used. This configuration is described in detail in U.S. Pat. No. 5,452,520. The magnet 3 in this device is centered in a housing or tube 2 by two ferrofluid rings 1,4 situated on either end of the magnet 3 and held by the magnetic field. There is no physical contact between the magnet 3 and the housing 2. The magnet 3 can slide freely because the ferrofluid rings 1,4 act as friction-less bearings inside the housing 2. The ferrofluid rings 1,4 also form low pressure seals between the magnet 3 and the walls of the housing 2. Accordingly, the ends of the housing 2 generally have small openings 5,6 to release any pressure build-up of air due to the motion of the magnet 3.
This device does not have the response time and calibration problems discussed above. However, as the magnet 3 moves, it leaves behind a thin layer of ferrofluid adhering to the wall of the housing 2 due to surface tension. Eventually, the ferrofluid in the ferrofluid rings 1,4 becomes depleted and the levitation force produced by the ferrofluid is reduced. If the length of the housing 2 is relatively long so that the magnet 3 excursion is long or the magnet 3 movement is rapid, a significant amount of the ferrofluid may be lost from the ferrofluid rings 1,4 during each travel of the magnet 3 through the housing 2, thereby rapidly depleting the ferrofluid rings 1,4. When the rings 1,4 are sufficiently depleted, magnet 3 will no longer be suspended resulting in failure of the device. Additionally, over time, ferrofluid may be lost through the vent holes 5,6 in the housing resulting in a short product life.
It therefore is desirable to reduce the response time of ferrofluid sensors while extending their life.